This invention relates generally to heating and/or melting, and refining a charge of aluminum in a direct fired furnace.
Using combustion to generate the heat for heating or melting a charge may have a deleterious effect on the charge. For example, oxidation of aluminum during direct heating results in the formation of dross. Another problem is that a significant amount of hydrogen is generated by the reactions of aluminum, water vapor and hydrocarbon species in the combustion flue gas. The hydrogen may dissolve into the molten charge, thus causing defects in the production of metal.
The prior art has addressed this problem to strip undesired gases by injecting an inert gas into the molten metal baths. This method may involve injecting nitrogen or argon through a submerged sparger to create a large number of small inert bubbles. The dissolved undesired gas diffuses into the inert gas bubbles and is then removed. This refining step is often conducted in a holding furnace where molten metal produced in a melting furnace is transferred and held prior to casting. When lower dissolved hydrogen contents are required, a special refining vessel is used to remove hydrogen more efficiently.
Those skilled in the art have addressed the problem of dross formation on the charge by providing a protective atmosphere over the charge surface between the furnace charge and the combustion reaction. The combustion gases are exhausted above the combustion reaction from the furnace so as to ensure that the combustion gases stay well removed from the charge surface. Important recent developments in this area are disclosed and claimed in U.S. Pat. No. 5,563,903 to Jebrail et al., U.S. Pat. No. 5,609,481 to Kobayashi and U.S. Pat. No. 5,961,689 all of which are incorporated herein by reference.
The ""903 patent provides that conventional protective atmosphere heating arrangement has produced acceptable results when the height of the top surface of the charge is low relative to the burner height or when the charge is molten. However, relatively high levels of nitrogen oxides (NOx) generation is associated with this system. Moreover, the fuel and oxidant consumption is relatively high as is the potential corrosion of refractory. walls and burner parts within the furnace is concern.
The ""481 patent provides for a direct-fired furnace wherein a charge-proximal stratum is established between the charge and combustion gases emanating from one or more burners oriented above the charge, wherein the protective atmosphere has a different oxidative effect upon the charge than does the combustion gases.
U.S. Ser. No. 09/033,608 discloses a method for providing heat to a large volume of furnace charge using combustion with a protective atmosphere therebetween, thus causing the reduced generation of NOx, using oxy-fuel combustion with a protective atmosphere therebetween, which enables the reduced consumption of fuel and oxidant as well as reducing the level of refractory corrosion.
There is no teaching or suggestion, however, for refining metal resulting from the presence of undesired gases during the process of simultaneously heating the metal and refinement therefor.
Accordingly, it is an object of this invention to provide a method for preventing undesired gases from dissolving into-molten metal and for removing dissolved undesired gases from the charge of a metal during melting of the charge in a direct fired furnace.
The above and other objectives, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is a process for producing molten aluminum having a low hydrogen content which comprises introducing an aluminum charge into a direct fired furnace to expose the charge to radiant heat emitted from above the charge; combusting a combustion mixture layer of fuel and oxidant to form a combustion layer which generates the radiant heat and combustion reaction products within the furnace; and introducing a hydrogen-free protective atmosphere into the furnace at a vertical distance below the combustion mixture layer. Also, a portion of a dissolved hydrogen-containing species from the molten aluminum by exposing a large portion of the molten aluminum to the protective atmosphere while the furnace is being fired with the combustion mixture.
In another embodiment, this invention is directed to a process for melting an aluminum charge into molten aluminum and simultaneously refining the molten aluminum to produce a refined molten aluminum having a low hydrogen content which comprises introducing the charge into a direct fired furnace to expose the charge to radiant heat emitted from above the charge; combusting a combustion mixture layer of fuel and oxidant to form a combustion layer which generates the radiant heat and combustion reaction products within the furnace; introducing a hydrogen-free protective atmosphere into the furnace at a vertical distance below the combustion mixture layer; and removing a portion of dissolved hydrogen from the molten aluminum by injecting an inert gas into the molten aluminum while melting the charge and refining the molten aluminum.
The inert gas, such as nitrogen or argon, may be injected into the molten aluminum prior to the complete melting of the charge into molten aluminum. A fluxing gas, such as chlorine, may be added to the inert gas. Also, the protective atmosphere contains less than 1%, and preferably less than 0.1% by volume of a hydrogen containing species.
For purposes of this invention, a xe2x80x9cchargexe2x80x9d is defined as the aluminum that is to be refined, which may include molten aluminum; a xe2x80x9ccombustion mixturexe2x80x9d is defined as the combustible mixture of an oxidant and fuel which is combusted to generate radiant heat and combustion reaction products within the furnace; and a xe2x80x9cprotective atmospherexe2x80x9d is defined as a stratum of gas, preferably inert and preferably situated between the charge and the combustion mixture, an example of which is the stratified atmosphere of the ""481 patent.